Apparent ionospheric total electron content variations prior to major earthquakes due to electric fields created by tectonic stresses

Growing evidence for ionospheric signatures of impending earthquakes comes from electron content measurements along slanted paths from GPS satellites to multiple ground stations located up to 500 km away from the epicenters. These slant total electron content (STEC) measurements deviate from the classic U‐shape pattern, starting about 40 min to over an hour before major earthquakes. Unlike other naturally occurring STEC fluctuations at midlatitudes, we show here that these earthquake‐induced deviations are simultaneous over a wide geographical area and do not propagate, thereby indicating a ground‐based origin. Prior to the 11 March 2011 Tohoku‐Oki earthquake ( M w 9.0), the deviations were as much as 10% of the undisturbed STEC. We argue that such deviations must be due to an electric field‐forced rise or fall of the main ionosphere with little change in the vertical electron density profile. Hence, “apparent” is used in the title. We show how stress‐related underground electric fields penetrate to 80 km altitude (above which penetration to the main ionosphere easily occurs) with magnitudes high enough to create STEC variations comparable to those observed. Since many thousands of GPS receivers exist worldwide, our theory suggests the possibility of early warning systems that could provide 10 to 20 min notice prior to large earthquakes, after allowing time for signal processing. This theory for prequake‐induced STEC fluctuations also explains the ground‐based ULF magnetic field data acquired by Fraser‐Smith et al. 40 min prior to the Loma Prieta earthquake.